The present invention refers to turbine pumps, and more particularly it relates to a method of manufacturing a stator stage for one such pump, in particular a turbomolecular pump.
The conventional stator stages of such pumps, even when they are equipped with radial blades, are generally manufactured by forming and milling techniques.
An application entitled xe2x80x9cTURBINE PUMP WITH A STATOR STAGE INTEGRATED WITH A SPACER RINGxe2x80x9d filed on May 31, 2002 in the name of the Applicant, discloses a turbomolecular pump comprising at least one stator stage integrated with a spacer ring, located in correspondence of a transition chamber at intermediate pressure. Such a stage comprises a disc with a plurality of radial blades, joined at their inner ends with the central disc region, and at their outer ends with the spacer ring. The blades form a very narrow angle with the disc plane, of the order of 10xc2x0, and overlap each other so as to leave very narrow channels therebetween, of the order of about 1 mm. Those values allow maintaining a molecular gas flow, in the direction from the pump inlet to the outlet, up to about 100 mtorr, that is 13,33 Pa, (it means that the pump can tolerate this pressure in the transition chamber) without losing the compression capacity of the upper stages.
Such a stator stage, where the blades are bound at both ends, cannot be manufactured by the milling and forming techniques usually employed for manufacturing conventional stator stages, since milling cannot be used for manufacturing inter-blade separation channels of reduced thickness, whereas forming does not allow manufacturing overlapping blades.
Therefore, the object of the present invention is to provide a method of manufacturing stator stages with partly overlapping radial blades, which blades have no free ends and are separated by very narrow channels.
The present invention promotes a method based on electrical discharge machining. A similar technique is already used in the field of turbine pumps to manufacture impellers and rotors, as disclosed in U.S. Pat. No. 5,188,514 entitled xe2x80x9cProcess For Manufacturing An Impeller By Electrical Discharge Machining And Articles So Obtainedxe2x80x9d issued Feb. 23, 1993. According to the teaching of this patent, a workpiece in which an impeller is to be obtained, is submitted in an apparatus for electrical discharge machining to the action of a tool comprising at least one plate in which radial slits are formed terminating in a central hollow area. Through a relative combined rotation and translation motion of the workpiece and the tool, which form the two electrodes of the apparatus, a radial blade of the impeller is manufactured for each slit. Yet the radial blades have a free end.
The object of the present invention is achieved through a method comprising a preliminary step in which a workpiece corresponding to the stator stage to be obtained is prepared and placed in an apparatus for electrical discharge machining of which the workpiece forms an electrode, in front of an electrical discharge machining tool forming the other electrode, and a relative combined rotation and translation motion of the tool and the workpiece is generated. The method is characterised in that, to manufacture a stator stage comprising a disc with a plurality of radial blades joined to the disc at both a radially inner end and a radially outer end and separated by very narrow channels, said tool comprises at least one plate equipped with at least one tongue-shaped elongated member that is joined at one end to the tool plate and extends along a peripheral edge of said plate in a plane inclined relative to the plate itself, at such a distance from the axis of said combined rotation and translation motion that the tongue always remains within said disc, so that, thanks to said relative combined rotation and translation motion of the tool and the workpiece, at least one channel, radially limited at both ends and having shape, size and inclination corresponding with those of said tongue element, is produced in the workpiece.
The features of the invention will become more apparent from the following description of a preferred embodiment, given by way of non limiting example and shown in the accompanying drawings.